This invention relates to a method of thermal mechanically treating cast austenitic heat resistant alloy structures to produce structures having superior strength and superior ductility at elevated temperatures and which also exhibit improved creep properties when exposed to carburizing or oxidizing environments at high temperatures.
Various industrial processes, especially chemical processes, create an insatiable demand for alloys and alloy products which can withstand higher and higher temperatures and environments deleterious to the alloys. Such deleterious environments include both carburizing and oxidizing environments, both of which are known to significantly affect plant performance and efficiency in many industrial processes. These effects are evidenced in such heat treatment equipment as, ethylene pyrolysis tubing, carbon dioxide and helium cooled nuclear reactors, coal processing plants, hydrocarbon reformers, and steam generators.
A variety of alloys and alloy products have been designed for application in such environments. More particularly, austenitic alloy steels exhibiting heat resistance and carburization resistance have been developed for use in pyrolysis furnaces for the thermal decomposition or organic compounds, such as the steam cracking of hydrocarbons. Generally, the pyrolysis furnace contains a series of heat-resistant alloy steel tubes in which the reaction occurs. The term "tube" as used herein also includes fittings, pipes and other parts used to contain carburizing and oxidizing materials at elevated temperatures.
When casting austenitic alloy steel into structures such as tubes, a microstructure develops which consists primarily of columnar grains oriented radially through the thickness of the tube wall. During high temperature service, this type of grain structure encourages the nucleation and propagation of cracks, which once initiated, have a tendency to run throughout the thickness of the structure. Because of this serious detriment, it is highly desirable to develop a method of treating such structures so as to inhibit the initiation and propagation of such cracks. Furthermore, it would be even more desirable to inhibit the initiation and propagation of such cracks while improving other high temperature properties such as creep and ductility.